If you're doing much with Termux or SSH to other systems (or roguelikes), you should probably look into Hacker's Keyboard (https://play.google.com/store/apps/details?id=org.pocketwork...). If you miss the autocompletion aspect, there are multiple companion dictionaries that you can load as well for various languages.

An on screen keyboard is fine for vim, at least for things light duty tasks. I use the one provided by my phone's vendor (it provides the number row) and Termux offers a row of common keys that usually gets the job done.

I also find that vim is easier to use for editing than Android editors. It is easier to use insertion, movement, and deletion commands instead of trying to tap on the right position or dragging to select.